What this calculator does: You enter your home’s square footage, ceiling height and insulation level, choose a tube diameter, and enter the air temperature drop you expect from the buried tubes. The calculator works out how much air an earth tube system would need to move, then gives the number of parallel tubes, a tube length, the spacing between tubes and an estimate of the cooling those tubes could deliver at the temperature drop you entered.

Who it’s for: Off-grid homeowners and builders curious about earth tubes as a low-power way to pre-cool a house, who want a first idea of how big such a system might be.

The answer it produces: Required airflow in CFM, the airflow of one tube at your chosen diameter, the number of parallel tubes, a length for each tube, the total pipe to lay, the spacing between tubes, a design air speed and an estimated cooling figure in Btu per hour and tons. It adds a warning when the array is large (more than six tubes) and another when you pick a hot and humid climate.

Important limitation: The tool sizes airflow from air changes per hour, and it picks the tube length from a fixed list according to your insulation choice. It does not predict the temperature drop. That is a number you enter, and the cooling estimate is only as good as that number. It knows nothing about your soil, and its only use of climate is to show a humidity warning. Earth tubes usually work as pre-cooling, not a replacement for air conditioning.

Size an Earth Tube System

An earth tube (also called an earth-air tunnel or Canadian well) draws outside air through a buried pipe, where the soil cools it before it reaches the house. It needs only a small fan, which is why it appeals to off-grid builders.

How the Calculation Works

When you press Calculate, the tool first checks your entries, then runs seven steps in your browser:

1. House volume. Square footage × ceiling height.
2. Required airflow. Volume × air changes per hour ÷ 60, giving cubic feet per minute (CFM). The air changes depend on your insulation choice: 4 for Poor, 3 for Average and 2 for Well-Insulated.
3. Airflow of one tube. The tool uses the diameter you choose and an air speed of 4 feet per second. That is the area of the tube × 4 × 60. For a 6 inch tube (0.196 square feet) it is about 47 CFM. A 4 inch tube carries about 21 CFM, an 8 inch tube about 84 and a 10 inch tube about 131.
4. Number of tubes. Required airflow ÷ the airflow of one tube, rounded up, never fewer than one.
5. Tube length. Not calculated. It is looked up from your insulation choice: 150 feet for Poor, 125 feet for Average and 100 feet for Well-Insulated. The length is the same whichever diameter you pick, and the tool tells you to bury the tubes at least 5 feet deep, with 6 to 8 feet a common practical range.
6. Total pipe and spacing. Number of tubes × length gives the total tube to lay. The tool recommends at least three tube diameters between parallel tubes, which is 1.5 feet for 6 inch pipe.
7. Estimated cooling. The airflow the tubes actually deliver (number of tubes × airflow of one tube) × 1.08 × the temperature drop you entered. The 1.08 is the standard rule of thumb for the heat carried by air, in Btu per hour per CFM per degree Fahrenheit. The result is also shown in tons, at 12,000 Btu per hour per ton.

The length point matters. It does not depend on your soil, your outdoor air temperature, your tube diameter or how much cooling you want. Natural Resources Canada’s design guidance treats pipe diameter, depth, length and air speed as choices to be modeled together, and notes that performance varies with soil type, depth, climate and operating schedule.

Inputs and Assumptions

And these are the assumptions built into the tool:

What the Result Means

• Required Airflow. The volume of air, in CFM, that would replace your house’s air 2, 3 or 4 times an hour. It is a ventilation figure. It is not a cooling load.
• Tube Diameter. The diameter you chose, with the airflow one tube carries at 4 feet per second.
• Number of Parallel Tubes Needed. How many tubes of your chosen diameter are needed to carry that airflow at 4 feet per second. Each one is a separate run of that length, so the pipe adds up quickly.
• Recommended Tube Length (each). The lookup length for your insulation level, not a calculated result.
• Total Tube to Lay and Spacing. The number of tubes × the length, and the minimum gap the tool recommends between parallel tubes.
• Design Air Velocity. The fixed 4 feet per second used in the calculation.
• Estimated Cooling Delivered. Btu per hour and tons at the temperature drop you entered. It counts only dry-air (sensible) cooling, and the tool notes that a real system delivers less on days when the soil around the tubes has warmed up.
• Warnings. A “Large array” warning appears when more than six tubes are needed. A “Humid-climate warning” appears if you chose Hot and humid.

Example Calculation

Take a 1,200 square foot house with 8 foot ceilings, average insulation, 6 inch tubes and the default 20°F temperature drop. The volume is 1,200 × 8 = 9,600 cubic feet. Three air changes an hour is 9,600 × 3 = 28,800 cubic feet an hour, or 480 CFM. One 6 inch tube carries about 47 CFM, so 480 ÷ 47 = 10.2, which rounds up to 11 tubes. Each is 125 feet long, for 1,375 feet of buried pipe, with at least 1.5 feet between tubes. Eleven tubes deliver about 518 CFM, and 1.08 × 518 × 20 is about 11,200 Btu per hour, roughly 0.9 tons of sensible cooling at that drop. Because there are more than six tubes, the tool also shows its large-array warning. Here is how the same house compares at the three insulation levels:

The diameter you choose changes the tube count a great deal. For the average-insulation house above:

Bigger tubes mean fewer trenches, but the tool uses the same length for every diameter and does not change the temperature drop you enter. In reality a larger pipe at the same air speed has less soil contact per unit of air, so the drop you can expect from a wide pipe is likely to be smaller. Lower the drop you enter if you choose a larger diameter.

Adjusting the example for the real world. First, what does the airflow actually deliver? The U.S. Department of Energy’s ventilation cooling guide says a fan moving 1,200 CFM at a 15°F temperature difference provides roughly 18,000 Btu per hour of cooling. The tool’s 1.08 factor gives 19,440 for that same case, a little higher. At the example’s 518 CFM, the tool gives about 5,600 Btu per hour at a 10°F drop, 8,400 at 15°F and 11,200 at 20°F. For comparison, DOE’s sizing guidance associates about 350 to 400 CFM with each 12,000 Btu per hour (one ton) of conventional air conditioning. So the earth tube here moves more air than a one-ton unit and removes less heat at the smaller drops, and the real drop depends on your soil and climate.

Second, look at the layout. Eleven parallel tubes need room. Natural Resources Canada recommends spacing pipes about three pipe diameters apart, or 1 meter (3.3 feet), which puts 11 tubes in a strip roughly 15 to 33 feet wide and 125 feet long. Raising the air speed to the 2 meters per second (about 6.6 feet per second) that NRCan prefers would cut the count from 11 to 7 tubes, but faster air gets less time in the soil and adds pressure the fan must overcome.

Third, look at depth. The tool says to bury the tubes at least 5 feet down, with 6 to 8 feet as a common practical range. NRCan’s guidance gives 1.5 meters (about 5 feet) as the minimum and 3 to 5 meters (about 10 to 16 feet) as the optimum, noting that soil temperatures are fairly constant below about 3 meters and that shallower installations see more seasonal variation and lower performance. The steady 50 to 59°F ground temperature that DOE describes applies at about 30 feet down, not at 6 to 8 feet.

Fourth, look at the temperature drop, because it is the number you have to supply. A test case on Build It Solar’s earth tube notes page measured air cooling from 105.4°F to 81°F, a drop of 24.4°F, through a 164 foot tube and produced about 0.31 tons of cooling with a 300 watt blower. By the same 1.08 formula, that cooling corresponds to only about 140 CFM of airflow, far less than the 518 CFM in the example above. So the tool’s 20°F default is a plausible drop for a long tube carrying a small airflow, and it may be optimistic for eleven tubes moving a whole house’s worth of air. Try 10 to 15°F for a cautious estimate.

How to Use the Result in a Real System

1. Treat it as a scale check. The output tells you whether an earth tube is practical at all for your house size. If it calls for a dozen tubes and over 1,000 feet of pipe, the large-array warning appears, and a smaller system for pre-cooling may be more realistic.
2. Reduce the house’s load first. Shade, insulation, air sealing and night ventilation cut the cooling you need, and they cost less than buried pipe.
3. Get a real load calculation. DOE’s HVAC sizing guide is built around a Manual J load calculation. An HVAC professional can tell you how many Btu per hour your house needs, which is what should drive the design.
4. Design for condensation. Natural Resources Canada says condensation is inevitable in most climates. Use smooth pipe, slope it at least 2% and provide a way to drain it, so water does not collect and grow mold.
5. Deal with radon. NRCan advises avoiding pipe with drainage slots or holes where radon may be present, and sealing around the tube so radon cannot enter. Check your area’s radon risk.
6. Choose a fan properly. A small inline fan of 10 to 30 watts can move air through a single tube at this scale, but a multi-tube array needs a fan sized for its total airflow and pressure drop. The added pressure of the tubes must be included in fan selection. Then size the power on the off-grid solar calculator and check how long a battery bank will carry it with the battery bank runtime calculator.
7. Plan a backup. Use the earth tube as a baseline and add active cooling for peak days, then put costs in the off-grid budget calculator.

Important Variables That Can Change the Result

• Soil type and moisture. Damp soil conducts heat better than dry sand. The tool cannot see either.
• Burial depth. Deeper is better, as the NRCan figures above show.
• Outdoor temperature and humidity. DOE’s ventilation cooling guide says moving outdoor air works in hot-dry climates and is not recommended where the air is humid. Cooling humid air in a tube also drives condensation.
• Pipe diameter, length and air speed. The tool lets you change the diameter but fixes the length and the air speed. Longer, slower, smaller tubes exchange more heat but move less air. NRCan gives 6 to 10 inch pipe, 100 to 130 feet as a practical minimum length and diminishing returns past about 230 feet.
• Pipe material and condition. Concrete conducts heat better than plastic, and NRCan advises against PVC where it connects to ventilation air because of off-gassing.
• The house itself. Sun exposure, windows, occupants, appliances and air leaks all set the cooling load.
• Operating pattern. NRCan notes that seasonal operation can saturate the soil with heat over a long hot spell.
• Fan power. Pressure drop rises with air speed, tube length and fittings.

Common Mistakes

• Reading the airflow as cooling capacity. CFM is air moved. The cooling depends on how much cooler that air is than your house, and the tool gets that drop from you, not from a model.
• Assuming the tube length is calculated. It is a lookup, so it may be too long or too short for your site.
• Burying at 6 to 8 feet and expecting a constant 50 to 60°F. Soil at that depth still follows the seasons.
• Skipping the drain and slope. Standing water in a buried pipe is a mold risk.
• Ignoring radon. Perforated pipe and leaky joints can let radon into the house air.
• Underestimating the digging. A dozen parallel trenches is a big excavation.
• Expecting to drop air conditioning entirely. NRCan describes earth tubes as essentially pre-cooling or pre-heating systems.
• Entering an optimistic temperature drop. The estimated cooling scales directly with it, so doubling the drop doubles the Btu per hour.
• Ignoring the humid-climate warning. Condensation inside buried pipe is a real design problem, not a footnote.
• Leaving the ceiling height blank or out of range. The tool shows an error instead of guessing, and it accepts 1 to 30 feet.

When This Calculator Should NOT Be Used

• To size a cooling system for a real house. It has no cooling load calculation. The Btu per hour figure is only the sensible cooling of the air at the temperature drop you enter, not what your house needs.
• As a construction or excavation plan. An engineer or experienced installer should design it.
• In humid or very hot climates. Condensation and limited cooling make a pure earth tube system a poor bet there. The tool shows a warning if you choose the humid option, but it does not change the numbers.
• Where soil is rocky, flooded or has a high water table. Deep trenching and drainage become hard.
• Where radon is a known problem, unless the design deals with it.
• To estimate energy savings or cost. It has no power, price or performance model.

DC Heat Pumps: The Active Cooling Option

Most home air conditioners and mini-splits run on AC power, so a battery’s DC power has to pass through an inverter first, and every conversion loses some energy. Pure-DC heat pumps skip that step by wiring straight to a 24 volt or 48 volt battery bank. How much they save depends on the two units being compared, and a figure of around 25% is often quoted. Check the rated efficiency of the specific models before you rely on any percentage.

For actual units, Full Battery sells 24 volt and 48 volt pure-DC heat pumps, mini-splits, window units and portable air conditioners from about 6,000 to 36,000 Btu per hour, built for solar and off-grid battery setups. Most off-grid homes get the best result by combining an earth tube as a low-power baseline with a DC heat pump for peak heat, so the compressor only runs when it is needed.

Our book No Inverter Needed: The Complete 48V Off-Grid DC Wiring Guide goes deeper on DC heat pumps, sizing and wiring for battery-based homes.

Related Honey Hen Calculators

• Off-Grid Solar Calculator: size the panels that will run a fan or heat pump.
• Battery Bank Runtime Calculator: see how long a battery bank will power cooling equipment.
• Off-Grid Heating Fuel Calculator: the cold-weather side of climate control.
• Off-Grid Budget Calculator: add the cost of cooling equipment and trenching.
• All Homestead Calculators: the full set of planning tools in one place.

Related Honey Hen Articles

• Earth-Sheltered Homes Passive Solar Design
• Off-Grid Passive Solar Home Materials to Use
• Energy-Efficient Off-Grid Appliances: 10 Picks for Homesteads
• 48V DC Hydronic Water Heater and Chiller for Off-Grid Homes

Authoritative Sources

• Natural Resources Canada: Earth to Air Thermal Exchanger (EATEX) Design Principles. Recommended pipe diameter, depth, length, air speed, spacing, slope, condensation, mold and radon guidance.
• U.S. Department of Energy Building America: Measure Guideline, Ventilation Cooling. How airflow relates to cooling, and the climate and humidity limits of ventilation cooling.
• U.S. Department of Energy Building America: Strategy Guideline, HVAC Equipment Sizing. Manual J load calculations, oversizing limits and airflow per unit of cooling.
• Build It Solar: Earth Tube Notes. A worked test case of the air temperature drop and cooling from a buried tube, and a warning about humid climates.
• U.S. Department of Energy: Geothermal Heat Pumps. Ground temperatures, and how heat pumps use the cooler ground in summer.

Earth Tube Calculator FAQ

Can an earth tube cool a house on its own?

Sometimes, in dry climates with moderate summer heat. Natural Resources Canada describes earth tubes as essentially precooling or preheating systems, and in more extreme or humid climates they work best as a baseline paired with active cooling.

Why does the calculator give me so many tubes?

Each 6 inch tube at 4 feet per second carries only about 47 CFM, and the tool sets airflow from air changes per hour of the whole house. A typical 1,200 square foot house comes out at 11 tubes. Choose a larger diameter for fewer tubes (4 tubes at 10 inches), though the tool does not adjust the temperature drop for you. Faster tubes are possible, but each option has trade-offs for cooling and fan power.

Does the calculator tell me how cool the air will be?

No. You enter the temperature drop, and the tool converts it into Btu per hour. It gives airflow, tube count and a lookup length, but it does not predict the outlet temperature, which depends on soil, depth, climate, air speed and pipe length.

What temperature drop should I enter?

The tool defaults to 20°F. A test case on Build It Solar’s earth tube notes page measured a 24.4°F drop through a 164 foot tube, but that was a much smaller airflow than a whole-house array. Use a lower figure, such as 10 to 15°F, for a cautious estimate, or if your soil is warm or dry, and treat the Btu per hour result as a rough guide.

How deep should an earth tube be buried?

Deeper than the tool’s 5 foot minimum and 6 to 8 foot practical range if you can manage it. Natural Resources Canada gives a minimum of about 5 feet to the top of the pipe and an optimum of about 10 to 16 feet.

Are there health risks with earth tubes?

Two to design for: mold from condensation that does not drain, and radon entering through perforated or leaky pipe. Natural Resources Canada recommends sloped smooth pipe with condensate drainage and a radon seal.

Do I need a professional to install an earth tube?

Many homesteaders build simple systems themselves, but slope, drainage, depth and radon sealing all matter. A consultation with a local geothermal or HVAC contractor can prevent an expensive redo.

This calculator provides general planning estimates, not an engineering design. Actual earth tube performance depends on soil type, moisture content, and local climate, which this calculator cannot account for — consult a qualified HVAC or geothermal professional before installation.